It is known that halogen-containing plastics have a tendency to undergo undesirable decomposition and breakdown reactions under thermal stress during processing or during long-term use. The breakdown of halogenated polymers, especially in the case of PVC, results in the formation of hydrochloric acid, which is eliminated from the polymer strand, resulting in a discoloured, unsaturated plastics having colour-imparting polyene sequences.
Especially in the case of moulded articles that are exposed to light, fluctuating temperatures or other external influences, as the period of use increases changes occur to the colour and material properties which may eventually lead to the moulded article's becoming unusable.
Various stabiliser systems have been proposed for stabilising halogen-containing polymers in respect of long-term damage caused by the action of light or heat. For example, systems are described in the prior art that comprise perchlorate salts for stabilising halogen-containing polymers. A problem that arises when perchlorate salts are used, however, is that those sensitive oxidising agents have to be handled very cautiously. For that reason, the user will have certain reservations about the use of perchlorate salts in stabiliser compositions. Various solutions have therefore been proposed by means of which such perchlorate-containing stabiliser compositions can be improved in respect of their handleability.
For example, EP-B 0 457 471 describes a stabiliser composition comprising perchlorate, calcium silicate and calcium carbonate. For the preparation of the described stabiliser compositions, an aqueous solution of sodium perchlorate is mixed with calcium silicate or with a mixture of calcium silicate and calcium carbonate. A problem that arises with the described stabiliser compositions is that they still contain relatively large crystallites of sodium perchlorate. Such crystallites can still constitute a hazard during handling. Furthermore, such stabiliser compositions cannot be distributed sufficiently homogeneously in a halogen-containing polymer. In addition, the presence of calcium silicate limits the stabiliser action in comparison with other calcium compounds. Moreover, calcium silicate exhibits a high Mohs hardness, so that processing apparatus can become permanently damaged.
DE 689 12 616 T2 discloses pulverulent flame retardants having a content of antimony pentoxide for halogen-containing vinyl resins. The specification describes compositions for flame retardants that comprise sodium perchlorate and also hydroxides or salts of an alkali metal. According to embodiments 2 and 4, the described compositions are dried and can then be ground in a pin disc mill.
DE 696 04 880 T2 relates to a stabiliser for halogen-containing resins and processes for the preparation thereof, a halogen-containing resin composition and a complex hydroxide salt. The stabiliser for the halogen-containing resins may comprise, for example, a complex hydroxide salt of a perchloric acid and inorganic metal hydroxides.
There was therefore a need for stabiliser compositions comprising a salt of a halogen-containing oxy acid, where the stabiliser compositions comprise the salt of the halogen-containing oxy acid as finely divided as possible.
The problem underlying the present invention is therefore to provide such stabiliser compositions and processes for their preparation.
The problems underlying the invention are solved by a stabiliser composition and a process for its preparation as described in the following text.